How accurate is the analyze complexity feature on leetcode?

Date : September 1, 2024
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# How Accurate Is the Analyze Complexity Feature on LeetCode?

If you’ve ever tackled a coding problem on LeetCode, you might have wondered about the accuracy of their space and time complexity analysis feature. Recently, I encountered an intriguing situation while solving a problem that had me questioning the results provided by the platform. I’d like to share my experience and thoughts on the accuracy of LeetCode's analyze complexity feature, particularly regarding space complexity.

## The Problem Encountered

I was working on the problem [Reverse Nodes in k-Group](https://leetcode.com/problems/reverse-nodes-in-k-group/) and implemented a solution that I believed utilized O(N) space complexity. To my surprise, LeetCode's analysis indicated that my solution had O(1) space complexity. This discrepancy made me doubt my understanding of space complexity and sparked a deeper investigation into how LeetCode evaluates solutions.

### My Solution

Here’s the code I implemented:

```python
class Solution:
    def reverseKGroup(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
        groupHeads = []
        groupSize = 0
        while head:
            if groupSize == 0:
                groupHeads.append(head)
            head = head.next
            groupSize += 1
            groupSize %= k
        prevHead = None
        if groupSize != 0:
            prevHead = groupHeads[-1]
            groupHeads.pop()
        groupHeads.reverse()
        for groupHead in groupHeads:
            prev = prevHead
            cur = groupHead
            for i in range(k):
                next = cur.next
                cur.next = prev
                if i == k - 1:
                    prevHead = cur
                prev = cur
                cur = next
        return prevHead

In this code, I created a list groupHeads to store the heads of the groups of nodes, which would imply that I was using additional space proportional to the number of groups I was processing.

Understanding Space Complexity

The confusion arose from how the space complexity was interpreted in this solution. While it seems that I was using O(N) space in terms of storing the heads of the groups, let’s break down the complexity:

O(N): If N is the total number of nodes in the linked list, storing every group head would indeed lead to O(N) space complexity.
O(1): However, LeetCode might consider the space complexity to be O(1) if they focus solely on auxiliary space usage. In this case, the space used by the pointers and the list’s internal storage could be regarded as constant space, especially since the algorithm does not allocate new space relative to the input size (N) but rather depends on the structure of the linked list itself.

Community Insights

I wasn’t alone in my confusion. Many users in the comments echoed similar sentiments. Some suggested that the space complexity should indeed be O(N/K), which could be a valid assertion depending on how we define the size of the groups and the maximum space utilized during the reversal process.

The Role of AI

Some users mentioned that the complex analysis might just be an output of AI algorithms, which could lead to discrepancies in understanding or interpreting space complexity. As algorithms evolve, the methods in which we analyze and compute complexities may also shift, leading to variations in expected results.

Conclusion

The accuracy of the analyze complexity feature on LeetCode certainly has its nuances. While the platform provides a helpful guide, it’s essential to approach these analyses with a critical mindset. Understanding the underlying principles of space and time complexity is vital, as they can vary depending on the interpretation and context of the solution.

In conclusion, I encourage fellow coders to not only trust the analysis provided by LeetCode but also to critically evaluate their own implementations. Engaging in discussions with the community can also help clarify these concepts and improve our understanding of algorithm complexities.

What are your thoughts on LeetCode’s complexity analysis? Have you encountered similar discrepancies in your coding journey? Let’s discuss in the comments below!

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## Related Posts
- [How to calculate time complexity and space complexity](../how_to_calculate_time_complexity_and_space_complex)
- [What is complexity for you? ](../what_is_complexity_for_you_)